Some implantable cardioverter defibrillators (ICDs) are configured to evaluate the morphology of an unknown cardiac EGM waveform in the detection and discrimination of cardiac arrhythmias. The morphology of the unknown waveform may be compared to a previously determined morphology template corresponding to a known waveform for classifying the unknown waveform based on whether the unknown waveform “matches” the known template. The usefulness of such comparisons are dependent on the quality of the sensed signal, the quality of the template, and the methods used to extract morphological features of the waveforms. Clipped QRS signals and double-peaked QRS signals are examples of sensed signals of unknown waveforms that are particularly challenging to classify accurately using template comparison algorithms. Methods are needed for accurate waveform classification based on morphology template comparisons.